Thermoelectric conversion process and apparatus



June 15, 1965 R. FORMAN 3,189,756

THERMOELECTRIC CONVERSION PROCESS AND APPARATUS Filed May 5, 1961 40 9 A m 30 l-AJ o 20 D it 40 g I o 4 \1) 10' 10 10 10 1o 10 4 L M mmvrox figs? RALPH FORMAN ATTORNEY 3,189,766 THERMOELECTRIC CONVERSION PROCESS AND APPARATUS Ralph Forman, Rocky River, Ohio, assignor to Union Carbide (Torporation, a corporation of New York Filed May 5, 1961, Ser. No. 108,104 10 Claims. (Cl. 310-4) The present invention relates generally to a process and apparatus for converting heat energy to electrical energy and, more particularly, to a process for converting heat energy directly to electrical energy by eifecting thermionic emission from a hot body while producing ions in the gas surrounding the hot body.

Heretofore, it has been proposed to convert heat energy to electrical energy by using a gas having a very low ionization potential with a hot electron-emitting material which has a work function higher than the ionization potential of the gas. Such a process is employed in the conventional cesium thermionic converter, wherein heat energy is converted directly to electrical energy by passing cesium gas, which has a very low ionization potential (3.8 ev.), over a hot tungsten cathode, which has a work function (4.6 ev.) higher than the ionization potential of the cesium gas and thus ionizes the cesium gas. The general operating principle for the cesium thermionic converter is that the ionized cesium produced by the hot filament neutralizes the space charge which is ordinarily responsible for inhibiting thermionic emission from the hot filament. Although the operating principle of such a cesium thermionic converter is a sound one, effective electron emitters usually have low work functions, and a relatively small number of gases have such low ionization potentials. Thus, relatively few gases are suitable for use in such devices. Also, gases having low ionization potentials are often chemically active and difficult to contain in a closed system.

More recently, it has been found that cesium gas can be ionized, even when the work function of the cathode is slightly below the ionization potential of the cesium, by employing high cathode temperatures (between about 1500 and about 3000 C.) and maintaining the cesium at a pressure between about 0.1 and about 2.0 mm. of mercury. However, the pressure of cesium required in such a device necessitates operating at relatively high ambient temperatures which, combined with the high chemical activity of cesium, makes construction of the device considerably more difiicult. Also, the cathode in such a device has a relatively short life.

It is, therefore, the main object of the present invention to provide a thermoelectric conversion process and apparatus wherein the thermionic work function of the cathode may be higher or lower than the ionization potential of the surrounding gas and wherein the gas to be ionized may be at a relatively low pressure, e.g., as low as 10* mm. of mercury.

Another object of the invention is to provide such a process and apparatus wherein the cathode has a relatively long life.

A further object of the invention is to provide an im proved process and apapratus for varying space charge efiects near a hot cathode so as to vary the thermionic emission therefrom.

, Other aims and advantages of the invention will be 2 apparent from the following description and appended claims.

In the drawings:

FIG. 1 is a schematic view of a preferred embodiment of the inventive apparatus;

FIG. 2 is a schematic view of another form of the inventive apparatus; and

FIG. 3 is a graph showing the output power obtained from the apparatus of FIG. 1 as a function of load resistance.

In accordance with the present invention, there is provided a process and apparatus for converting heat energy to electrical energy by disposing a cathode and an anode in a rare gas medium containing at least one ionizable gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, the cathode having a thermionic work function greater than the thermionic work function of the anode and being electrically connected to the anode through an external load circuit; maintaining said rare gas at a pressure such that the mean free path therein for atoms of said ionizable gas is less than the distance between said cathode and anode; and increasing the temperature of said cathode and the pressure of the ionizable gas until the concentration of gas ions is sufiiciently high to susbtantially neutralize the space charge surrounding the cathode, while continuously maintaining the temperature of the cathode above the temperature of the anode and sufficiently high to effect thermionic emission from the cathode.

Heretofore, it has been shown that high pressure gases are ionized at comparatively low temperatures by a thermal ionization process. The theoretical equation expressing this ionization eifect is:

wherein n, is the ion concentration; n is the neutral gas atom concentration; g g,, and g are the statistical weights for the electron, ion, and neutral atom, respectively; h is Plancks constant, In is the electron mass; e is the electronic charge; k is Boltzmanns constant; T is the temperature of the gas; and V, is the ionization potential of the gas.

It has also been shown that the aforedescribed process of thermal ionization is in competition at elevated temperatures with a surface ionization process, which is described by the theoretical expression:

egi (2) o wherein the only new quantity l/ is the work function of a hot surface in the gas. At low pressures of gas, where the mean free path of the gas molecules is high, the process given by Equation 2 predominate and is mainly responsible for the ion formation. At high pressures, where the mean free path of the gas molecules is low, the process given by Equation 1 predominates.

Because of the low ionization potentials of the alkali metal gases and the alkaline earth metal gases, a relatively larger ion concentration can be produced by placing small concentrations of such metal gases in a high pressure rare gas. Indeed, it can be shown that the thermal ionization due to Equation 1 is much greater in a high pressure rare gas containing a minor impurity of alkali metal gas or alkaline earth metal gas than is the surface ionization Table l Ambi- Ambi- Ambient Cs ion ent K ion cnt Na ion Partial Temp. Cone. at Temp. Cone. at Temp. Cone. at Pressure of for Cs this Presfor K this Presfor Na this Pres- Alkali Gas to get sure to get sure to get sure (mm. of Hg) Pres- (ions/ Pres- (ions/ Pres- (ions/ sure cmfi) sure cmfi) sure cm?) 107 2. 0X10" 160 3. X10 229 l. 6X10 69 6. 3X10 119 l.1; 188 5. 0x10 45 2. 0X10 88 3. 5X10 157 1. 6 1O 6. 3X10 (i3 1.1)(10 124 5.0X10 0 2. 0X10 4O 3. 5X10 97 1. 6X10 The results shown in Table I are to be compared with the results of Table II, which shows the ion concentrations obtainable by the surface ionization process of Equation 2 under the same conditions.

Table 11 Am- Am- Ambient Cs ion bient K ion bieut Na ion Temp. Cone. Temp. Cone. Temp. Cone. Partial Presfor Cs at this for K at this for Na at this sure of Alkali to Oh- Pressure to Ob- Pressure to Ob- Pressure (mm. of Hg) tain (ions/ tain (ions/ tain (ions/ Prescmfi) Presemi Pres- 0111.

sure sure sure It is obvious that the ion densities given in Table I are much greater than the ion densities for the same partial pressures in Table II. Thus, when an alkali metal gas or alkaline earth metal gas is placed in a high pressure rare gas, a relatively low partial pressure of the metal gas is required to produce a relatively large ion concentration around the hot cathode. Accordingly, the ambient temperature of the metal gas is also relatively low, and the problems arising from the chemical activity of the metal gas are considerably reduced.

In order to effect thermal ionization of the alkali metal or alkaline earth metal gas, the pressure of the rare gas must be increased until the mean free path therein for the atoms of metal gas is less than the distance between the cathode and anode. As the gas pressure is increased, the mean free path of the gas atoms is decreased, and gas atoms bouncing off the hot cathode make numerous collision with other gas atoms. Thermodynamically, the hot gas atoms have a finite probability of being ionized, given by Equation 1.

Although the relatively high pressure of the rare gas provides a very low means free path for the atoms of alkali metal or alkaline earth metal gas, the peculiar electron cross-section properties of the rare gases, especially neon, also ensure a long mean free path for slow energy electrons emitted by the cathode of the thermionic converter. In other words, the metal gas atoms in the vicinity of the hot cathode make frequent collisions with other gas atoms as a result of their low mean free path and are ionized by the process described by Equation 1, whereas the electrons emitted from the cathode experience very few collisions in traversing the distance between the cathode and anode because of the collision cross-section prop erties of the rare gas. The relatively high pressure of the i gas surrounding the cathode also inhibits the evaporation of cathode material, thus substantially extending the life of the cathode.

The only requirement of the cathode temperature is that it be sufiiciently high to effect thermionic emission therefrom and to ionize a sufficient number of the alkali metal or alkaline earth metal gas atoms to substantially neutralize the space charge surrounding the cathode. When the ion concentration has been increased sufficiently to substantially neutralize the space charge surrounding the cathode, thermionic emission therefrom proceeds uninhibited.

When the cathode temperature required to achieve effective thermionic emission is between about 1500 and 1709 IL, a concentration of alkali metal gas or alkaline earth metal gas in the range of parts per billion to one part per hundred is usually sufiicient to provide an ion concentration high enough to substantially neutralize the space charge surrounding the cathode. The pressure of the alkali metal gas or alkaline earth metal gas under such conditions should normally be in the order of 10- to 10- mm. of mercury, and the pressure of the rare gas should be between about one mm. of mercury and atmospheric pressure. Of course, the specific magnitude required for any of the aforedescribed process variables depends on the magnitude of each of the other variables. For example, if a lower cathode temperature were employed, the pressure and/or the concentration of the alkali metal or alkaline earth metal gas could be increased.

In addition to the ions of alkali metal or alkaline earth metal gas, there may be some rare gas ions in the gas mixture produced by thermal ionization. Also, there may be ions of cathode material due to collisions between evaporated cathode material and gas molecules. However, as mentioned above, the amount of cathode material evaporated decreases as the gas pressure increases, thus extending the life of the apparatus.

The preferred gas for use in the ionizable portion of the gas mixture is cesium, but the other alkali metals (sodium, potassium, lithium, and rubidium), or the alkaline earth metals (calcium, strontium, and barium), may also be employed. The rare gas comprising the remainder of the gas mixture surrounding the cathode is preferably neon, but any other rare gas, such as argon, krypton, or xenon, may also be employed. The concentration of the ionizable gas in the rare gas must be such that ionization thereof substantially neutralizes the space charge surrounding the cathode.

The cathode employed in the p esent process and apparatus is preferably porous tungsten containing imbedded barium aluminate or barium-strontium carbonate. The barium aluminate cathode has a work function of 2.12 electron volts and is capable of emitting current densities of about 10 amperes/cm. at l500 K. However, any other suitable electron-emitting material may be employed, regardless of whether its work function is greater than, equal to, or less than the ionization potential of the particular alkali metal gas employed. In order to obtain an output voltage from the inventive converter, the oathode must have a thermionic work function greater than that of the anode and must be connected to the anode by electrically conductive means and an external load. The anode temperature must be continuously maintained below the temperature of the cathode.

The inventive process and apparatus will now be described in greater detail by referring to the drawings.

A schematic view of a preferred embodiment of the inventive apparatus is shown in FIG. 1. The apparatus comprises an annular copper base lll between an upper chamber 12 and a lower chamber 14. The upper chamber 12 is formed by a stainless steel sleeve 13 brazed to the copper base 1%, a kovar sleeve 15 secured to the sleeve l3, and a glass envelope 17 secured to the kovar sleeve 15. Chamber 12 is filled with a rare gas supplied through tube 18 from any convenient source (not shown).

The minor impurity of alkali metal or alkaline earth metal gas is supplied through tube 23 from the breakseal bottle 19. The metal gas vapor, formed from metal 21, is admitted to the tube 18 and chamber 12 by magnetically raising the metal slug 29 and dropping it so as to break the glass projection 25.

A cathode mounting 22 is secured to an annular flange on the upper surface of the copper base 10. The pocket in the upper surface of the cathode mounting 22 contains the cathode pellet 24, which preferably consists of a porous tungsten plug containing imbedded barium aluminate or barium-strontium carbonate. Surrounding the cathode mounting 22 is an anode in the form of cylindrical shell 26 and ring 27. The anode is suspended from supporting member 28 which is secured to the glass envelope 17 at 30.

Since the cathode mounting 22 and the copper base It) are electrically conductive, the external lead 32 for the cathode 24 is afiixed directly to the outer surface of the copper base 10. The external lead 34 for the anode is connected directly to the cylindrical shell 26. The two leads 32 and 34 are externally connected through load 36.

Supported within the cathode mounting 22 is an alumina-coated tungsten heater coil 40, which is heated by an electrical current passed through conductors 42 and 44. The only requirement on the heater coil 40 is that it be sufficient to heat the cathode pellet 24 to the temperature required to effect thermionic emission therefrom and to ionize a sufficient number of gas atoms of the particular alkali metal or alkaline earth metal gas employed to substantially neutralize the space charge surrounding the cathode. In order to form a vacuum between the heater coil 40 and the cathode mounting 22, kovar sleeve 47, with a glass envelope 48 secured thereto, is brazed to the bottom of the copper base so as to form the chamber 14 which can be evacuated through tube 49.

Another form of the inventive apparatus is shown in FIG. 2. This embodiment comprises a pair of concentric conductive cylinders 51 and 52 held in place by insulating end rings 57 and 58 so as to define an annular chamber 53. Affixed to the inner cathode support cylinder 51 is a cathode sleeve 56 of impregnated tungsten or nickel. The outer cylinder 52 comprises the anode of the system. The chamber 53 is exhausted through tube 55 and then filled through the same tube with 10 to 100 mm. of neon containing cesium gas as a minor contamination. The cathode 50 is connected to an external load circuit through conductor 59, while the anode is connected to the same load circuit through conductor 60. The cathode 50 is heated by passing an appropriate hot fuel or gas through the tubular passageway 57.

In an example of the inventive process as carried out in the apparatus of FIG. 1, the upper chamber 12 was first exhausted and then filled with neon at a pressure of 40 mm. of mercury. The cathode, a porous tungsten plug containing imbedded barium aluminate (work function of about 2.12 ev.), was heated to a temperature of about 1500 K. The cathode was spaced about 5 mm. from the anode, which was maintained at a temperature below that of the cathode simply by being located closer to the relatively cool outer wall of the device. The anode was cesium coated nickel, which has a work function below 2.12 ev.

With only neon in the chamber 12, the short circuit current (R o) was measured as 4 l() amperes. The glass projection 25 on the side tube 19 was then broken to admit cesium gas into the chamber 12. With the cesium gas at a pressure of 10 mm. of mercury and the neon still at a pressure of 40 mm. of mercury, the short circuit current was measured as 0.1 ampere. The vastly increased short circuit current indicated that the cesium gas was being ionized and neutralizing the space charge surrounding the cathode.

The device was then connected to an external load resistance (R and the power output was measured and recorded at various values of load resistance. The power output values are shown in FIG. 3 as a function of the load resistance. The peak of the curve in FIG. 3 indicates that the internal impedance of the thermoelectric generator was about 10 ohms. This curve remained substantially unchanged as the pressure of the cesium gas was varied from about 10 to about 10* mm. of mercury.

While various specific forms of the present invention have been illustrated and described herein, it is not intended to limit this invention to any of the details herein shown, but only as set forth in the appended claims.

What is claimed is:

1. A process for thermoelectric conversion comprising disposing a cathode and an anode in a gaseous medium which comprises a major amount of a rare gas and a minor amount of at least one ionizable gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, said cathode having a thermionic Work function greater than the thermionic work function of said anode and being electrically connected to said anode through an external load circuit; maintaining said rare gas at a pressure such that the mean free path therein for atoms of said ionizable gas is less than the distance be tween said cathode and anode; and increasing the temper ature of said cathode and the pressure of said ionizable gas until the concentration of gas ions is sufliciently high to substantially neutralize the space charge surrounding said cathode, while continuously maintaining the temperature of said cathode above the temperature of said anode and sufficiently high to effect thermionic emission from said cathode.

2. A process for thermoelectric conversion comprising disposing a cathode and an anode in a rare gas medium containing at least one ionizable gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, said cathode having a thermionic work function greater than the thermionic work function of said anode and being electrically connected to said anode through an external load circuit; maintaining the pressure of said ionized gas between about 1() and 10- millimeter of mercury; maintaining said rare gas at a pressure such that the mean free path therein for atoms of said ionizable gas is less than the distance between said cathode and anode; and increasing the temperature of said cathode sufiiciently high to effect thermionic emission therefrom and to produce a concentration of gas ions sufficiently high to substantially neutralize the space charge surrounding said cathode, while continuously maintaining the temperature of said anode below the temperature of said cathode.

3. A process for thermoelectric conversion comprising disposing a cathode and an anode in a rare gas medium containing at least one ionizable gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, said cathode having a thermionic work function greater than the thermionic work function of said anode and being electrically connected to said anode through an external load circuit; maintaining the concentration of said ionizable gas in said rare gas between about parts per billion and about one part per hundred; maintaining said rare gas at a pressure such that the mean free path therein for atoms of said ionizable gas is less than the distance between said cathode and anode; and increasing the temperature of said cathode sufficiently high to effect thermionic emission therefrom and to produce a concentration of gas ions sufficiently high to substantially neutralize the space charge surrounding said cathode, while continuously maintaining the temperature of said anode below the temper-ature of said cathode.

4. A process for thermoelectric conversion comprising disposing a cathode and an anode in a rare gas medium containing at least one ionizable gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, said cathode having a thermionic Work function greater than the thermionic work function of said anode and being electrically connected to said anode through an external load circuit; maintaining the pressure of said ionizable gas between about 10- and about millimeter of mercury; maintaining the pressure of said rare gas between about one millimeter of mercury and atmospheric pressure; and increasing the temperature of said cathode sufiiciently high to eiiect thermionic emission therefrom and to produce a concentration of gas ions sufliciently hi h to substantially neutralize the space charge surrounding said cathode, while continuously maintaining the temperature of said anode below the temperature of said cathode.

5. A process for thermoelectric conversion comprising disposing a cathode and an anode in a rare gas medium containing a minor impurity of at least one ionizable gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, said cathode having a thermionic work function greater than the thermionic work function of said anode and being spaced a predetermined distance from said anode and electrically connected to said anode through an external load circuit; maintaining said rare gas at a pressure such that the mean free path therein for atoms of said ionizable gas is less than the distance between said cathode and anode; and increasing the temperature of said cathode and the pressure of said ionizable gas until the concentration of gas ions is sulficiently high to substantially neutralize the space charge surrounding said cathode, while continuously maintaining the temperature of said cathode above the temperature of said anode and sufiiciently high to efiect thermionic emission from said cathode.

6. A process for thermoelectric conversion comprising disposing a cathode and an anode in neon containing cesium gas, said cathode having a thermionic work function greater than the thermionic Work function of said anode and being electrically connected to said anode through an external load circuit; maintaining said cesium gas at a pressure between about l() and about 10 millimeter of mercury; maintaining said neon at a pressure between about one millimeter of mercury and atmospheric pressure; and increasing the temperature of said cathode sufiiciently high to effect thermionic emission therefrom and to produce a concentration of cesium ions sufiiciently high to substantially neutralize the space charge surrounding said cathode, while continuously maintaining the temperature of said anode below the temperature of said cathode.

7. An apparatus for thermoelectric conversion comprising a cathode and an anode disposed in a gaseous medium which comprises a major amount of a rare gas and a minor amount of at least one ionizable gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, said cathode having a thermionic work function greater than the thermionic work function of said anode, said cathode being electrically connected to said anode through an external load circuit; means for increasing the pressure of said rare gas until the mean free path therein for atoms of said ionizable gas is less than the distance between said cathode and anode; means for increasing the temperature of said cathode and the pressure of said ionizable gas mixture until the concentration of gas ions is sufiiciently high to substantially neutralize the space charge surrounding said cathode, the temperature of said cathode being sufficiently high to effect thermionic emission therefrom; and means for continuously maintaining said anode at a temperature below the temperature of said cathode.

8. An apparatus for thermoelectric conversion comprising a cathode and an anode disposed in a rare gas containing at least one ionizable gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, said cathode having a thermionic'work function greater than the thermionic work function of said anode, said cathode being space-d a predetermined distance from said anode and being electrically connected to said anode through an external load circuit; means for maintaining the concentration of said ionizable gas in said rare gas between about 180 parts per billion and about one part per hundred; means for increasing the pressure of said rare gas until the mean free path therein for atoms or" said ionizable gas is less than the distance between said cathode and anode; means for increasing the temperature of said cathode until the concentration of gas ions is sufficiently high to substantially neutralize the space charge surrounding said cathode, the temperature of said cathode being sufficiently high to effect thermionic emission therefrom; and means for continuously maintaining said anode at a temperature below the temperature of said cathode.

9. An apparatus for thermoelectric conversion comprising a cathode and an anode disposed in a rare gas containing at least one ionizabie gas selected from the group consisting of the alkali metal gases and the alkaline earth metal gases, said cathode having a thermionic work function greater than the thermionic Work function of said anode and being electrically connected to said anode through an external load circuit; means for maintaining said ionizabie gas at a pressure between about l0 and about 1() millimeters of mercury; means for maintaining the pressure of said rare gas between about one millimeter of mercury and atmospheric pressure; means for increasing the temperature of said cathode sufficiently high to effect thermionic emission therefrom and to produce a concentration of gas ions sufficiently high to substantially neutralize the space charge surrounding said cathode; and means for continuously maintaining said anode at a temperature below the temperature of said cathode.

it An apparatus for thermoelectric conversion comprising a porous tungsten cathode containing imbedded barium aluminate and an anode disposed in a mixture of neon and cesium gas, said anode having a thermionic work function less than the thermionic work function of said cathode, said cathode being electrically connected to said anode through an external load circuit; means for maintaining the concentration of said cesium gas in said neon between about parts per billion and about one part per hundred; means for increasing the pressure of said neon to between about one miliimeter of mercury and atmospheric pressure; means for increasing the temperature of said cathode to about l500 K; and means for continuously maintaining said anode at a temperature below the temperature of said cathode.

References Cited by the Examiner UNITED STATES PATENTS 2,510,397 6/50 Hansell 3104 X 2,759,112 8/56 Caldwell. 2,975,320 3/61 Knauer. 2,980,819 4/61 Feaster 3 l0-4 FOREIGN PATENTS 797,872 7/58 Great Britain.

OTHER REFERENCES Publication: Thermionic Energy Converter, by Hernqvist, Kanefsky and Norman, RCA. Review, volume 19, No. 2, pages 244, 251 to 258.

Publication: Evaporation of Tungsten Under Various Pressures of Ar on, Physical Review, volume 31, February i928, page 263.

MILTQN O. HIRSHFIELD, Primary Examiner.

DAVID X. SLKNEY, E "amiizer. 

1. A PROCESS FOR THERMOELECTRIC CONVERSION COMPRISING DISPOSING A CATHODE AND AN ANODE IN A GASEOUS MEDIUM WHICH COMPRISES A MAJOR AMOUNT OF A RARE GAS AND A MINOR AMOUNT OF AT LEAST ONE IONIZABLE GAS SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METAL GASES AND THE ALKALINE EARTH METAL GASES, SAID CATHODE HAVING A THERMIONIC WORK FUNCTION GREATER THAN THE THERMIONIC WORK FUNCTION OF SAID ANODE AND BEING ELECTRICALLY CONNECTED TO SAID ANODE THROUGH AN EXTERNAL LOAD CIRCUIT; MAINTAINING SAID RARE GAS AT A PRESSURE SUCH THAT THE MEAN FREE PATH THEREIN FOR ATOMS OF SAID IONIZABLE GAS IS LESS THAN THE DISTANCE BETWEEN SAID CATHODE AND ANODE; AND INCREASING THE TEMPERATURE OF SAID CATHODE AND THE PRESSURE OF SAID IONIZABLE GAS UNTIL THE CONCENTRATION OF GAS IONS IS SUFFICIENTLY HIGH TO SUBSTANTIALLY NEUTRALIZE THE SPACE CHARGE SURROUNDING SAID CATHODE, WHILE CONTINUOUSLY MAINTAINING THE TEMPERATURE OF SAID CATHODE ABOVE THE TEMPERATURE OF SAID ANODE AND SUFFICIENTLY HIGH TO EFFECT THERMOINIC EMISSION FROM SAID CATHODE. 